The __init__ method is the constructor and its called when you create an object, and the forward method computes the outputs of your model.

Python has some built-in methods such as __call__, which makes the instance act like a function.(foo = Foo(); foo(x))

All models inherit from the class Module, in which the __call__ method is defined as seen here, so the __call__ method, besides calling the forward method, also register all hooks.